Dump trucks and other machines for hauling payloads of granular material are equipped with bodies or beds that are designed to limit the material that may fall off or out of the bed during transport. The designs of these machines must take account for steep grades and, as a consequence, the use of tail-gates or tail-barriers becomes necessary. The effectiveness of a tailgate may be limited as a payload of granular material is typically loaded high on the bed and it may not be practical to design a tailgate as high as the top of the payload. Further, large tailgates are heavy, which limit the weight of the payload that the truck or machine may safely carry.
An alternative to large tailgates is a cover or tarp draped over the top of the payload. Many cover designs are available, including mechanisms that automatically deploy the cover. However, covers are only practical when the granular material is lightweight and will not tear or cut the cover. Covers are not practical for heavy dump trucks that carry material like rocks that weigh several kilograms to several hundred kilograms. Further, covers are often not preferred because of the time it takes to deploy a cover, which consumes valuable haulage time. For example, in a mining operation where a truck may be constantly hauling granular material from one location to another, the time consumed by deploying a cover each time the truck is loaded would reduce the number of loads per day the truck can haul.
Because large tailgates and covers are usually impractical for heavy dump trucks, heavy dump trucks are designed with deep beds. A deep bed can limit material spillage as long as the material is properly distributed on the bed. However, deep beds require a severe, near vertical tilt during the dumping process. A severe, near vertical tilt requires a hoist mechanism with a larger and heavier piston and, therefore, a heavier hoist mechanism. Such a heavier hoist mechanism can limit the weight of the payload the truck can safely carry. Thus, there may be a tradeoff between the depth of the bed, which enables larger payloads, and the tilt angle required to dump the payload from the bed, which may ultimately limit the weight of the payload.
The overall size of a bed presents an additional limitation. If the granular material is lightweight, the granular material may be loaded very high in an attempt to reach the maximum haulage capacity of the truck or vehicle. The higher the material is loaded, the more likely it is to spill, especially on steep grades. As a result, for lighter materials, the beds must be designed deeper and larger and, therefore heavier. Eventually a point may be reached where making the bed any larger would require a payload weight reduction.
When granular materials are poured onto a horizontal surface, such as a truck bed, a conical pile will form. The internal angle between the surface of the pile and the horizontal surface is known as the angle of repose (γ). For a conically-shaped payload disposed on a truck bed, the angle of repose is the angle between the surface of the pile and the truck bed. Materials with a low angle of repose form flatter piles than materials with a high angle of repose. The angle of repose represents the steepest slope possible relative to the truck bed, or, when the granular material on the slope face is on the verge of sliding. The angle of repose is related to the density, the surface area of the granular particles, the shapes of the particles and the coefficient of friction of the granular material.
When hauling a payload of granular material up a grade, if the granular material is loaded into a conical pile and the surface of the top of the pile is at or near the angle of repose, the angle of the grade will destabilize the payload, possibly causing spillage. Vibrations can also destabilize a conical pile of granular material. If the granular material includes sharp or large rocks, spillage can present a safety hazard to following vehicles or trucks. As truck tires are very expensive, the economics of a mining or hauling operation can also be compromised.
Therefore, there is a need for an improved system and method for loading dump truck beds and other hauling vehicle beds with granular material that maximizes the payload while minimizing spillage during both flat hauls and hauls proceeding up a grade.